Hello Don.
I would say, first of all would be interesting to know in wich resort are you going to stay. Most of them have a.dive center allready from where they can organize trips to Cozumel, cenotes or reef diving in Playa.
My recommendation is that you do a few dives in playa before heading to Cozumel or Cenotes.
The reason. I would.recommend you to get some experience before those trips.
Cozumel has strong currents and some of its best dive sites are a bit deep. If you strait go straight to Cozumel you risk that they take you to less spectacular dive sites.
Cenotes, normally dive centers ask for some experience or at least a few dives with them before taking you there.
Anyway I strongly recommend you to try them. It is something you will never experience anywere else in the world. For begginers i recommend you Chack Mool or Dos Ojos.
Aswell now is supposed to be bull shark season in playa del carmen so for sure you find a dive center that can take you for a Bull Shark excursion.
I hope this helps you a bit.
I strongly recommend you tk.read this article:
Top Diving Destinations: Riviera Maya